1. Field of the Invention
The present invention relates to a ferromagnetic resonator formed of ferromagnetic thin film and suitable for use in microwave devices, and particularly, to a ferromagnetic resonator designed for use in suppressing spurious response.
2. Description of Prior Art
By use of the liquid phase epitaxial growth technology for growing a garnet magnetic film on the gadolinium-gallium garnet (GGG) substrate that has become popular recently through the development of magnetic bubble memory devices, it is possible to make an yttrium-iron garnet (YIG) thin film with satisfactory crystallinity. By forming the YIG thin film into disk or rectangular shape through the selective etching process, and utilizing its ferromagnetic resonance property, microwave devices can be constructed. Application of usual photolithography facilitates the manufacturing process, and yet a high productivity is promised, since a sheet of GGG substrate yields a large number of devices. Moreover, because of it being a thin film material, microwave integrated circuits (MICs) can easily be realized using microstrip lines for transmission lines.
As has been known in the art, microwave devices utilizing ferromagnetic resonance are advantageous in compactness and sharpness of response, and YIG single crystalline spheres have been used in practice to make such microwave devices. The YIG single crystalline sphere is advantageous in that it is hardly excited in magnetostatic modes and a unique resonance mode can be obtained by a uniform precession mode. However, the YIG single crystalline sphere has shortcomings in manufacturing and productivity, and therefore, formation of ferromagnetic resonator using the YIG thin film has been desired.
The YIG thin film has had a problem of being apt to excite in many magnetostatic modes even if it is placed in a uniform RF magnetic field, due to its nonuniform internal DC magnetic field. Magnetostatic modes of a disk-shaped ferrimagnetic specimen with a DC magnetic field applied perpendicularly to the specimen surface is analyzed in an article in Journal of Applied Physics, Vol. 48, July 1977, pp. 3001-3007. Each mode is expressed by (n,N).sub.m, i.e., the node has n modes in the circumferential direction, N nodes in the radial direction, and m-1 nodes in the thickness direction. When the high-frequency magnetic field is applied uniformly to the whole area of specimen, the (1,N).sub.1 series becomes the major magnetostatic mode. FIG. 1 shows, the measured result of ferromagnetic resonance in a disk shaped thin film specimen measured in the 9 GHz cavity, indicating the excitation in many magnetostatic modes of (1, N).sub.1 series. When this specimen is used to form a microwave device such as a band-pass filter, its major resonance mode, i.e., mode (1,1).sub.1 is used, and in this case all other magnetostatic modes cause spurious response.